Cardiac muscle fibrillation is the rapid and asynchronous contraction of individual muscle fibers in the heart. The result is a slightly quivering and non-functional heart muscle. When fibrillation occurs within the lower chambers of the heart or ventricles, blood flow ceases and, if not corrected within minutes, death of the patient will result. Fibrillation occurring only in the upper chambers of heart or atria results in a diminution of cardiac output that may be symptomatic to the patient. Other forms of cardiac dysrhythmia include ventricular or supraventricular tachycardia, which are very rapid orgailized/synchronous muscle fiber contractions that impair cardiac output to lesser or greater degrees dependent on cardiac refill times and preload pressures.
Implantable cardiovertex and defibrillator (ICD) systems attempt to treat of cardiac dysrhythmias by passing through the heart muscle a cardioversion or defibrillation countershock, depending on the type of cardiac dysrhythmia diagnosed. An objective of the cardioversion or defibrillation countershock is to immerse as much of the myocardium as possible within the electrical field generated by the countershock. The countershock is a truncated capacitive discharge of electrical energy that generally ranges from 0.1 to 5.0 joules for cardioversion and from 5 to 40 joules for defibrillation of the ventricles.
U.S. Pat. No. 4,774,950, the disclosure of which is incorporated herein by reference, discloses an embodiment of an abdominally implanted ICD. U.S. Pat. No. 5,405,363, the disclosure of which is also incorporated herein by reference, describes embodiments of a pectorally implanted ICD.
Conventional abdominally implanted ICDs include a metal housing that floats with respect to the internal electronic circuitry. The housing is not physically secured to the internal electronics and is tied to the battery ground through a high impedance, typically 200K ohms. This is desirable to prevent the housing from acting as an electrode and thereby siphoning off current that should flow between the two defibrillation electrodes. Preventing the housing from acting as an electrode at an abdominal location is especially desirable, to avoid diverting current from the heart. Even though the housing is essentially floating, the housing acts as a shield against electromagnetic interference and protects the internal electronic circuitry from picking up induced pulses from stray electromagnetic fields.
Unlike abdominally implanted ICD's, pectorally implanted ICD's, such as described in U.S. Pat. No. 5,405,363, typically use the housing as one of the electrodes. When the housing is used as an electrode, implanting the housing in the pectoral region on the patient's left side and inserting a transvenous electrode into the patient's right ventricle will cause the defibrillation current to be directed along a very desirable vector. This results in a lower defibrillation threshold, that is, a lower minimum energy to produce successful defibrillation.
A common approach to delivering shocks with an ICD is to divide the shock into more than one phase. With biphasic waveforms, the polarity of the second phase is opposite to that of the first phase. It has been demonstrated that many biphasic waveforms can successfully defibrillate with consistently lower voltage and energy requirements than monophasic waveforms of the same duration. With biphasic waveforms the two electrodes of the defibrillator change polarities in mid-pulse.
When using a pectorally implanted housing as an electrode for biphasic-waveform shocks, the housing must be switched from one polarity of the output capacitor to the opposite polarity. In this case, the housing of the device cannot serve as an electrode simply by tying the housing to one of the battery electrodes, as it could be done for a monophasic pulse. When the housing is configured as an electrode for pectorally implanted ICD delivering a biphasic waveform, the housing acts as a large, first plate of a capacitor, and the other conductors in the circuitry of the ICD act as a second plate of the capacitor. When the housing voltage suddenly changes, voltages and currents are induced on nearby conductors. These induced currents and voltages potentially can be of sufficient magnitude so as to introduce unwanted logic signals or switching signals in the circuitry of the ICD, which have the possibility of causing circuit malfunctions and consequent harm to the patient.
For circuits with conductors having significant length, or lying adjacent to the housing, this kind of capacitive coupling effect is magnified. Such conductors are common in ICDs having interconnect wiring between various parts of the circuit, for example. These conductors often are in a medium known as a "flex tape," wherein conductive paths are printed on a flexible dielectric film that is wrapped, folded and bent to reach various points of the circuit to which connections must be made. Often, the most convenient route for the flex tape to reach these various points is around the outside of the electronics core, that is, near the inside surface of the housing thereby increasing the likelihood of undesirable capacitive coupling.
One way to address the problem of noise imparted onto the low voltage circuitry in a pectorally implanted ICD is to shield the circuitry so as to reduce the capacitive coupling between the housing and the circuitry. This is described in U.S. Ser. No. 08/486,759, filed Jun. 7, 1995, which, as stated above, is assigned to the assignee of the present invention and which has been incorporated by reference. This solution has the drawback of potentially increasing the total volume of the internal circuitry of the ICD and thereby limiting the overall size of the device. The shield current itself can inductively couple stray signals into the circuitry due to its imperfect conductivity and grounding.
As ICDs become smaller, especially pectorally implanted ICDs, the interconnect conductors and electronics are positioned nearer to the inside housing surface of the device. When the housing of the ICD is used as a switchable electrode, it is no longer possible to couple the housing to ground, for example, to allow the housing to serve as a capacitor coupling shield. Consequently, capacitive coupling problems that have not been experienced in the past are impacting the design and operation of new ICDs. These unanticipated capacitive coupling problems limit the miniaturization and effective operation of such devices.